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Lessons in Scaling Wireless Charging

The New York Times ran a piece on Saturday about a company, WiTricity Corporation, that promises to license near-field remote-charging technology for portable devices later this year.  The technology is said to be based on research conducted at MIT .

The abstract in the paper says, in part: “The method demonstrated in this paper allows a fixed-load receiver to be moved to nearly any position and/or orientation within the range of the transmitter and still achieve a near constant efficiency of over 70% for a range of 0-70 cm.”  (That's a little over two feet.)

Back in February, a press release from Stanford University  said that a research team there, “has designed a high-efficiency charging system that uses magnetic fields to wirelessly transmit large electric currents between metal coils placed several feet apart. The long-term goal of the research is to develop an all-electric highway that wirelessly charges cars and trucks as they cruise down the road.”

(The Stanford results to date have been published in Applied Physics Letters. In the press release, the Stanford researchers acknowledge the MIT work, adding some information I didn’t know:  “The MIT researchers have created a spinoff company that's developing a stationary charging system capable of wirelessly transferring about 3 kilowatts of electric power to a vehicle parked in a garage or on the street,” which is pretty ambitious, and quite a step beyond the 2007 work.

Meanwhile, at Stanford, the group there has wondered, “if the MIT system could be modified to transfer 10 kilowatts of electric power over a distance of 6.5 feet – enough to charge a car moving at highway speeds. The car battery would provide an additional boost for acceleration or uphill driving.”  The object is to install a series of coils under the roadway that would successively interact with tuned coils in the vehicle as it drives along.  Considering that we can’t presently keep road crews from digging up and destroying the coils used for traffic-light control, I think there are some human-factors issues that will have to be resolved before that plan is practical.

Meanwhile, for a dose of reality, I spent some time last Friday talking to IDT about magnetic-resonance charging on the scale that WiTricity is talking about.

As of this morning (March 12), IDT is announcing two chips; the IDTP9030, which lives in a charger mat or base, and the IDTP9020, which lives in your smart phone or other portable device.   Together, they comprise a wireless power transmitter and receiver solution that meets the Wireless Power Consortium’s (WPC) Qi (“chee”) interoperability standard. (Microcontrollers and memory in each chip mean that they can also support other proprietary formats and added features.)  On the power side, the “30” chip has a synchronous half-bridge converter to drive the transformer primary, and the “20” has a synchronous full-bridge rectifier on the secondary.

The Qi standard is important.  There is a complex handshake that must take place before the transformer can be powered up.  It’s simple to detect that something conductive has been placed in proximity to the primary, but what if it’s a pack of cigarettes with a foil wrapper?  You don’t want power to be applied before it’s plain that the external device is a compatible portable device, and for efficiency and power management reasons, you want to remove power when the battery in the portable device has reached its maximum state of charge. (To download the Qi standard, go here.)

The Qi standard makes it possible to mix and match transmitters and receivers, but IDT points out that while it can guarantee delivering 5 W to the system if its receiver is used with a generic Qi transmitter, if both chips are used together – in the charging platform and in the powered device – they can guarantee  up to 7.5 W.  Other key points include a small bill-of-materials and a small footprint.

Looking at what IDT has done in inductive chargers for portable devices provides an idea of what it will take to scale-up near-field magnetic-resonance charging for vehicular charging – once somebody invents a way to keep road maintenance crews from digging-up the coils.
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